The back pressure over 3500 RPM reaches 3 psi- that's a 50% increase and definitely over the manufacturer's recommendation. How many boat owners typically run slower than that on their way back to shore at the end of the day on larger lakes and longer river systems?

If you’re a math wiz and/or an engineer, you’re probably going to like this article and the resources we’ve linked to. However, if you find yourself getting stuck (or bored) with the info below, here are the key take-aways:

1. The factory exhaust pipe diameter is usually a good choice for most vehicles.

2. The muffler manufacturers are doing all the math for us – no need to reinvent the wheel. If they say it will work for your vehicle, it will probably work for your vehicle.

3. We’ve got an easy-to-read exhaust system size table that is good for quick calculations.

Breaking Down The Problem
While we’re not going to go through and list out all the formulas and calculations you need to figure this exactly, we will break down the problem, explain how you would go about figuring things out scientifically, and then leave you with some good quick-and-dirty exhaust system math as well as some interesting links.

The science goes like this…

1) Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases
Conservation of mass, right?

2) To calculate the volume of air the engine takes in, we multiply the displacement of the engine by the engine RPM and then divide by two (it takes two full revolutions for the engine to exhaust it’s entire air volume). We then convert that to volume to mass.

3) To make the calculations easy, you want to assume that combustion is perfect, i.e. there aren’t any byproducts, any unburned fuel, etc. It’s easier to assume perfect combustion and then “back in” to the actual numbers using an estimate after the fact.

4) Since you’re assuming perfect combustion, it’s easy to figure out how much fuel mass is added to the exhaust.

5) Once you know the mass of the exhaust gas, you just figure out how much volume that mass would occupy. Of course, you have to adjust for expansion due to the high exhaust gas temperature.

That’s it! Of course, when you sit down to figure it, you’ll find that getting a good scientific estimate takes a lot of work (which is why we don’t bother with it here).

Quick and Dirty Exhaust System Math
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.

Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).

Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:

If you’re a math wiz and/or an engineer, you’re probably going to like this article and the resources we’ve linked to. However, if you find yourself getting stuck (or bored) with the info below, here are the key take-aways:

1. The factory exhaust pipe diameter is usually a good choice for most vehicles.

2. The muffler manufacturers are doing all the math for us – no need to reinvent the wheel. If they say it will work for your vehicle, it will probably work for your vehicle.

3. We’ve got an easy-to-read exhaust system size table that is good for quick calculations.

Breaking Down The Problem
While we’re not going to go through and list out all the formulas and calculations you need to figure this exactly, we will break down the problem, explain how you would go about figuring things out scientifically, and then leave you with some good quick-and-dirty exhaust system math as well as some interesting links.

The science goes like this…

1) Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases
Conservation of mass, right?

2) To calculate the volume of air the engine takes in, we multiply the displacement of the engine by the engine RPM and then divide by two (it takes two full revolutions for the engine to exhaust it’s entire air volume). We then convert that to volume to mass.

3) To make the calculations easy, you want to assume that combustion is perfect, i.e. there aren’t any byproducts, any unburned fuel, etc. It’s easier to assume perfect combustion and then “back in” to the actual numbers using an estimate after the fact.

4) Since you’re assuming perfect combustion, it’s easy to figure out how much fuel mass is added to the exhaust.

5) Once you know the mass of the exhaust gas, you just figure out how much volume that mass would occupy. Of course, you have to adjust for expansion due to the high exhaust gas temperature.

That’s it! Of course, when you sit down to figure it, you’ll find that getting a good scientific estimate takes a lot of work (which is why we don’t bother with it here).

Quick and Dirty Exhaust System Math
Easy Way To Estimate: Your intake system needs to flow 1.5 CFM per engine horsepower, and your exhaust system needs to flow 2.2 CFM per engine horsepower.

Good Way To Estimate: Take engine RPM x engine displacement, then divide by two. This is the intake volume. Use this same volume of air for the exhaust system, but then correct for thermal expansion (you need to know exhaust temps to figure things out).

Exhaust Pipe Size Estimate: A good section of straight pipe will flow about 115 CFM per square inch of area. Here’s a quick table that shows how many CFM each common pipe size will flow, as well as the estimated max horsepower for each pipe size:

The table above is probably over-estimating pipe size, but you can see that a 400 hp vehicle with a dual exhaust system only needs 2 1/4 – 2 1/2 inch pipes. Anything larger is overkill.

I would think the intake air temperature would be needed, too. The air density change in a hard-running engine with cold air temps is much greater than in hot air.

Re: back pressure in cars- I was driving this weekend and saw an early-'80s Impala or Caprice and when I heard the sound of the exhaust as he stood on it (and was going absolutely nowhere) made me laugh because it was clearly a stock exhaust system with as much back pressure as GM could build into it.

If back pressures are higher, Manifold absolute pressure will be higher as well. The ECU would compensate for that. With a "Speed Density" fueling system, like our boats have, the MAP sensor measures intake manifold pressure, in order to make fueling adjustments, based on the load, and flow of the engine. If you restrict the exhaust flow, you reduce the efficiency of the "pump", and the inlet side will have a corresponding change in pressures.

Not saying that the corresponding changes will be as "spot on" as the normal parameters on an engine without FAE, however, it does compensate for things like that.

O2 sensors will only make microscopic changes to an engines fueling, and ONLY under a sustained, low throttle, cruising condition. The O2 sensors are ignored, by the ECU, under any kind of load above 1/8-1/4 throttle. They are also ignored at idle. They would not help compensate for an FAE at all.

Same goes in a car engine- You are cruising along at 50mph on a nice straight, flat, highway. Your foot is just barely pressing the throttle, and holding it steady. At this time, the ECU will take a look at your O2 readings (goes into closed loop), and make 1-3% changes in the short term fueling of the engine. As soon as you either let off, or put any kind of load, above 1/4 throttle, the ECU immediately ignores O2 readings (switches to open loop), and uses MAF, (or MAP sensor, depending on fueling type. Some engines run both), Intake air temp, engine temp, RPM, and throttle position, to make adjustments to fueling.

Because of the way boats are used, and how they are constantly under heavier loads, an O2 sensor is ignored 99.9% of the time.

You are right about the O2 sensor, however, like I said above, they absolutely can compensate for changes in air flow. They absolutely have to, otherwise, you would have to rewrite the ECU, with another program, every time you changed elevation, or even temperature outside....... Not to mention, the flow characteristics, of an engine, change over its lifetime.

Flow has EVERYTHING to do with it. Changes in elevation, outside temperature, engine temperature, backpressures, and intake restrictions (dirty air filter), all effect the FLOW of an engine. They are designed to deal with a wide range of changes. And sure, there is a limit to what it can compensate for, but they leave a pretty healthy cushion.

With all of that said, no way did the FAE take that connecting rod out.....

Your description is mildly inaccurate in the way engine sensors are used. ECM's are constantly monitoring MAF and O2 sensors, not just at a stable cruise. O2 sensors have the capability to make ~30% +/- adjustments depending on application along with MAF are the most accurate way the ECM along with other ECM inputs will adjust the air/ fuel mixture for the perfect stoichiometric mixture of 14.7:1. If you have an increase in exhaust backpressure ie. plugged exhaust or collapsed tubes etc. the pressures in the intake manifold vacuum will be less not higher and the MAP will see this and adjust the ECM output your correct there but when no restriction is involved it's like the don't ask policy, nobody knows. When a "cat-back" system is added to a vehicle, the ECM has no idea it's installed thats why most cat-back mfg's recommend an ECM update to adjust the fuel mixture. The whole purpose of the O2 sensor is to monitor the exhaust gas for the amount of oxygen that is present then gets reported to the ECM. While you are mildly correct in elevation, temperature etc. thats the job of upstream sensors not the O2.

__________________
/spôrk/
a spoon-shaped eating utensil with short tines at the tip

Your description is mildly inaccurate in the way engine sensors are used. ECM's are constantly monitoring MAF and O2 sensors, not just at a stable cruise. O2 sensors have the capability to make ~30% +/- adjustments depending on application along with MAF are the most accurate way the ECM along with other ECM inputs will adjust the air/ fuel mixture for the perfect stoichiometric mixture of 14.7:1. If you have an increase in exhaust backpressure ie. plugged exhaust or collapsed tubes etc. the pressures in the intake manifold vacuum will be less not higher and the MAP will see this and adjust the ECM output your correct there but when no restriction is involved it's like the don't ask policy, nobody knows. When a "cat-back" system is added to a vehicle, the ECM has no idea it's installed thats why most cat-back mfg's recommend an ECM update to adjust the fuel mixture. The whole purpose of the O2 sensor is to monitor the exhaust gas for the amount of oxygen that is present then gets reported to the ECM. While you are mildly correct in elevation, temperature etc. thats the job of upstream sensors not the O2.

I am sorry, but that is completely incorrect. First of all, what boat motor have you ever seen with a MAF?? I have never seen one. They use a "speed/density" calculation, not Mass Air Flow. .......Second, the O2 sensors ARE only looked upon, for data, under "Closed loop" conditions. And these engines only go into closed loop under a steady, light throttle, cruise. Narrow band O2 sensors are not capable of measuring the oxygen offset under any other condition. Which is why nobody would ever tune an engine with a narrow band O2 sensor. Have you ever data logged an engine, and looked at the short term fuel corrections for O2 readings??? They only have the ability to modify short, and long term fueling under Closed loop conditions. They may be able to adjust up to 30% of the engines "allowable" fuel trim.....that does not mean a 30% change. It means that the engine has an allowable range, and that under closed loop (and only under closed loop) the O2 sensors can modify the fuel trim, up to 30% of that range (the other 70% is reserved changes in air density). Like I said before, when you accelerate, or are above a light throttle, your ECU is in "Open loop", and is ignoring O2 readings, and is also not using any short, or long term fuel corrections, based on the last O2 readings that it looked at.

And the MAP reading will be Higher, as it is a reading of engine vacuum.....

I am sorry, but that is completely incorrect. First of all, what boat motor have you ever seen with a MAF?? I have never seen one. They use a "speed/density" calculation, not Mass Air Flow. .......Second, the O2 sensors ARE only looked upon, for data, under "Closed loop" conditions. And these engines only go into closed loop under a steady, light throttle, cruise. Narrow band O2 sensors are not capable of measuring the oxygen offset under any other condition. Which is why nobody would ever tune an engine with a narrow band O2 sensor. Have you ever data logged an engine, and looked at the short term fuel corrections for O2 readings??? They only have the ability to modify short, and long term fueling under Closed loop conditions. They may be able to adjust up to 30% of the engines "allowable" fuel trim.....that does not mean a 30% change. It means that the engine has an allowable range, and that under closed loop (and only under closed loop) the O2 sensors can modify the fuel trim, up to 30% of that range (the other 70% is reserved changes in air density). Like I said before, when you accelerate, or are above a light throttle, your ECU is in "Open loop", and is ignoring O2 readings, and is also not using any short, or long term fuel corrections, based on the last O2 readings that it looked at.

And the MAP reading will be Higher, as it is a reading of engine vacuum.....

I understand how a speed density system and a mass air flow system works, and can not compare both because the word O2 was mentioned and all the hyperbolies started to flow. (as I think I was as was other posts) and yes MAF is not used in this application. Think we both have the point on manifold vacuum, just describing it from different points. Substitute "MAP is high" for "manifold vacuum is low".

Either way the FAE did not cause the engine failure.

__________________
/spôrk/
a spoon-shaped eating utensil with short tines at the tip

And the MAP reading will be Higher, as it is a reading of engine vacuum.....

If exhaust back pressure is increased, how would the MAP sensor read higher vacuum when that's generally altered by the throttle plate opening and closing. Or, do you mean the intake will scavenge more easily, so it would seem like the throttle is more open?

If exhaust back pressure is increased, how would the MAP sensor read higher vacuum when that's generally altered by the throttle plate opening and closing. Or, do you mean the intake will scavenge more easily, so it would seem like the throttle is more open?

Simple way to look at what I mean- What number is higher? -5, or -10.....

If the exhaust is restricted, the MAP will read a higher reading, (but it is a lower amount of vacuum). Due to the fact that some engines are forced inducted, and that some can also have a + MAP reading, depending on elevation, and temperature conditions..... The readings for a MAP sensor are looked at that way. Basically, less vaccum= Higher pressure.

If there was no such thing as forced induction, or changes in barometric pressure, it would probably be looked at the other way...... But vacuum is a negative value.

Simple way to look at what I mean- What number is higher? -5, or -10.....

If the exhaust is restricted, the MAP will read a higher reading, (but it is a lower amount of vacuum). Due to the fact that some engines are forced inducted, and that some can also have a + MAP reading, depending on elevation, and temperature conditions..... The readings for a MAP sensor are looked at that way. Basically, less vaccum= Higher pressure.

If there was no such thing as forced induction, or changes in barometric pressure, it would probably be looked at the other way...... But vacuum is a negative value.

Opening the throttle decreases the vacuum seen by the MAP sensor, so clarity is needed when discussing this. The throttle opening results in higher voltage return from the MAP sensor but it's not really helpful to comment about forced induction in this thread because so far, I haven't seen universal agreement WRT the terms and a 2 atmosphere MAP sensor is needed, not the ones used in these engines. Vacuum is negative pressure, so less vacuum should be referred to as less vacuum, not 'more pressure'.

Also, while elevation does affect barometric pressure, barometric pressure should be the term used because air pressure is what the MAP sensor is measuring when the key is turned to ON (even briefly), not elevation. 29.60" Hg at sea level is the same as at 5000', so elevation is basically irrelevant- these run rich enough that erring on the lean side within the program's parameters won't cause engine failure unless a series of circumstances happen to coincide. If the air being less dense at 5000' causes a problem, it will be a rich condition, anyway.

This aside, if FAE has tested these extensively and boat manufacturers have used them without failures, it appears to work, even if the engine manufacturers recommend not exceeding 2psi back pressure and these are at 3psi above 3500 RPM, which is 50% high.